The invention concerns a sintered silver-iron material with 0.5 to 20% iron by weight and 0 to 5% by weight other metal, oxide, nitride, and/or carbide additives, the remainder being silver, which is useful for electrical contacts, and a process for producing this material.
Electrical switching contacts include stationary and moving conducting surfaces that make and/or break electric circuits. The choice of materials depends on the application. Common contact materials include palladium, silver, gold, mercury, and various alloys. Plated and overlaid surfaces of other metals such as nickel or rhodium are used to impart special characteristics such as long wear and arc resistance or to limit corrosion.
Materials for electrical switching contacts can be prepared by powder metallurgy. Powder metallurgy is the process of manufacturing articles from metallic powders. Powder metallurgy involves three main processes. First, the metal or alloy powder must be prepared. Second, the powder must be compacted in order to have sufficient strength for handling. Third, the resulting compacted material must be heated at a high temperature in a controlled atmosphere for such a time that the density of the compact increases to the desired value.
The purpose of the powder compaction process is to bring the individual powder particles into very intimate contact so that metal-to-metal bonding takes place. This compaction confers a small amount of mechanical strength and facilitates the mass transfer that must occur later during sintering to produce densification. Sintering involves compressing metal particles into a solid under heat, but at a temperature below their melting point.
After compaction, the material is heated at a high temperature in a controlled atmosphere. During sintering, the voids within the compact are progressively eliminated by atom movements and eventually a dense compact is produced practically free from porosity.
Sintering times vary and the sintering temperature is generally not less than two thirds of the melting point of the metal in degrees Kelvin. Sometimes the temperature is much more than this.
Contact materials for use in .he electrical power industry must have high resistance to burning [contact erosion], low sticking force [low contact welding force] and low contact resistance. Silver-nickel composite has proved useful for switches in air for low-voltage systems with switched currents of less than 100 A. It has high resistance to burning and very good over-heating behavior. One disadvantage of the material, though, is that nickel, especially in dust form, can be harmful to humans. For this reason, there have been various suggestions for use of iron as an alternative to nickel.
Electrical contact materials which contain iron, nickel, chromium, and/or cobalt along with silver are known from Japanese patent application 79/148109. Materials having the composition Ag and Fe (10%) have high resistance to contact welding while retaining good electrical conductivity.
Silver-iron materials for special contact applications are 41 also described in the journal "Materials and Methods," Vol. 44, No. 3, September 56, pages 121-126. Silver contact materials containing 0.001 to 1% nickel, iron, molybdenum, cobalt, chromium, titanium and/or vanadium, along with silver and the component that is oxidized are known from German patent application 11 53 178. German patent application 11 06 965 describes a process for producing dense sintered shapes from silver with 5 to 50% of at least one of the metals vanadium, tantalum, chromium, molybdenum, tungsten, iron, cobalt or nickel. They can also be used as contact materials.
Silver-iron materials have not been used more widely because they tend to form coatings during switching, thus producing high contact heating. None of these materials match the favorable properties of the sintered silver-nickel material.
All the known silver-iron materials for electrical contacts are made from standard commercial iron powder with a carbon content of less than 0.05%, so that they are relatively soft. The sintering is usually carried out in a hydrogen-containing atmosphere, particularly in nitrogen-hydrogen mixtures.
Therefore it is an objective of this invention to develop a sintered silver-iron material containing 0.5 to 20% by weight iron and 0 to 5% by weight of other metallic, oxidic, nitridic, and/or carbidic additives, the remainder being silver, which is useful for electrical contacts. Such a material should have little tendency to weld, low contact resistance, and high resistance to burning, so as to have a long lifetime. Its properties should approach as nearly as possible those of the known silver-nickel contact materials. A further objective is to develop a process for producing such a material.